The invention disclosed and claimed herein pertains generally to a means and method for determining an unknown amount of potential energy which is stored in a capacitor or capacitive electrical network in the form of electric charge. More particularly, the invention pertains to the measurement of the energy which may be stored in a capacitor of unknown capacitance, wherein the voltage across the unknown capacitor is also unknown and may not be easily determined. Even more particularly, the invention pertains to a means and method for determining whether or not a specified minimum amount of energy is stored in a capacitor of unknown capacitance.
In various electrical and electromechanical systems, a specified minimum amount of energy must be stored in a capacitor in order to effectively perform a task required for system operation. In a certain type of rocket, for example, a capacitor storing a specified minimum amount of energy is required for a mechanism employed in the rocket for rapidly releasing the first stage of the rocket from the remainder at a given point during rocket flight. At the given point, the energy stored in the capacitor is used to detonate explosive charges, which are positioned to blow away connecting bolts and provide the required rapid release.
As is well known the potential, or capacitive, energy U which is stored in a capacitor is given by the expression U=1/2CV.sup.2, where C is the capacitance of the capacitor and V is the voltage thereacross. However, due to imperfections in real world capacitors, the energy stored in a capacitor coupled to a voltage source may be significantly less than the nominal values of the capacitor and voltage source would indicate. For example, if the capacitor had an unknown internal resistance, the energy actually provided thereby would be below the anticipated amount due to loss across the internal resistance. If the internal resistance was substantial, the capacitor could be unable to deliver a required minimum amount of energy to a system dependent thereupon. Other unknown or unexpected effects which reduce the storage capability of a capacitor include voltage leak, a decrease in capacitance, and total failure of the capacitor.
Consequently, it becomes necessary to measure the energy stored by an energy storage capacitor to determine whether or not sufficient energy can actually be provided thereby for effective system operation.
While various techniques are presently available for determining capacitance, some of such techniques may not be well suited for measuring the capacitance of energy storage capacitors contained within the interior of an elaborate electrical or electromechanical system. Some of such present techniques may also be ineffective where the voltage across the capacitor is unknown and may not be readily measured. In any case, a measuring system requiring the measurement of both the capacitance of and voltage across an energy storage capacitor would be needlessly complicated, since the important determination is whether or not the capacitor is able to store a specified minimum amount of energy.
One presently available technique for measuring the energy stored in a capacitor requires the use of an oscilloscope. An energy storage capacitor is discharged through a fixed resistor, and a curve showing discharge voltage is displayed on the oscilloscope. However, the required oscilloscope is comparatively expensive, and may be too immobile to be used for testing energy storage capacitors which are located within an electrical or electromechanical system rather than in a laboratory. In addition, human calculations are generally necessary as part of such technique, in order to determine stored energy from the displayed data, whereby an operator having a degree of observational and mathematical skill is required, and the possibility of human error is introduced.
Another presently available technique requires the use of an ergmeter, an energy storage capacitor being discharged through the heating element of the ergmeter. However, the ergmeter is sensitive to ambient temperature, and employs a thermistor which requires frequent calibration.
In techniques employing either an oscilloscope or an ergmeter, further errors may occur as a result of line loss or contact resistance.